With ever increasingly stringent noise reduction requirements at airports, noise reduction systems for use with jet engines on private and commercial aircraft are becoming increasingly important. The turbulent mixing of primary-secondary and/or secondary-ambient streams is essentially what produces the broadband jet noise emitted from a jet engine. Takeoff conditions are particularly important since the emitted noise power levels increase with increasing velocity difference between the mixing streams.
Present day noise reduction systems used on commercial aircraft typically involve some form of stationary (i.e., static or fixed geometry) devices, which are often referred to in the industry as “chevrons” or “lobe-mixers”. These fixed devices are typically employed at a downstream edge of an exhaust nozzle and formed so that they protrude into the flow path of the exhaust gas emitted from the jet engine. This causes an intermixing of the exhaust gas with the airstream adjacent the exhaust nozzle. This intermixing serves to reduce the broadband noise generated by the jet engine over a wide range of operating conditions.
The drawback with the above-described, present day chevrons is that such devices, being fixed in their positions, protrude into the exhaust flow at all times during operation of the jet engine. This, of course, generates drag and a loss of thrust. This is important because noise reduction is typically needed only during takeoff of an aircraft and not during cruise conditions. Thus, with present day noise reduction systems that employ fixed chevrons or other like fixed elements, a tradeoff occurs between the needed noise reduction and the desire to avoid the loss of thrust during cruise conditions. As will also be appreciated, noise reduction systems employing fixed chevrons or other like elements which cannot be moved out of the flow path of the exhaust gasses of a jet engine will result in increased fuel burn during cruise operations, thus contributing to increased operating expense for the given aircraft.
It would therefore be highly desirable to provide a noise reducing system for a jet engine used on an aircraft that incorporates a plurality of flow-altering components that can be placed in the flow path of the exhaust flow created by the jet engine during a takeoff condition, but which can be readily retracted out of the flow path once the aircraft reaches a cruise condition. A noise reduction system employing chevrons or other like elements that could be designed to move in a manner that drives disturbances that are closely coupled to the naturally unstable frequencies of the mixing process would be highly desirable in enhancing the initial mixing of the streams without generating large-scale fluid motion. Such a noise reduction system would provide the desired degree of noise reduction during takeoff conditions but would not negatively affect the thrust generated by the engine during cruise conditions, and thus would not negatively impact the amount of fuel required for a given flight.